22 
BULLETIN 906, IT. S. DEPARTMENT OF AGRICULTURE. 
Saugus, Calif., is 80 feet high and was built to protect a 16-inch rein- 
forced concrete pipe that runs gradually up grade for over 2,000 feet. 
The standpipe is made of 12-inch riveted steel pipe, supported by a 
steel windmill tower. 
Open boxes of concrete are built to about 15 feet in height and act 
as relief stands and at the same time are used as diversion boxes or 
for measuring water over weirs. 
Where pressures are too great to permit the installation of a stand- 
pipe, it is common to use air drums to relieve back pressure. Air 
drums are chambers that are partly full of air, the elasticity of the 
air preventing excessive pressures on the pipe. Air chambers have 
been used successfully in some installations, but there are so many 
cases of the air chamber being waterlogged at a critical time that a 
PAPER RING SET INS IDE 
THIMBLE TO PREVENT 
ADHERING 
^^COPPER THIMBLE SET 
IN PIPE 
Fig. 5. — Expansion joints for high pressure. 
number have been replaced by relief stands. Some air drums are 
equipped with an automatic air pump that keeps the chamber filled 
with the proper proportion of air, but as any such apparatus is liable 
to get out of order, the air drum may be Avorthless when needed the 
most. There are some instances on record where air pumps have de- 
livered an excess of air which has caused large bubbles of air to col- 
lect in the main. 
Relief valves have also been used. These are so adjusted that an 
excess of pressure will open them and let out enough water to bring 
down the pressure in the pipe. Eelief valves are not in great favor, 
however, as they may stick under high pressures or blow off under 
small variations of pressure which occur when a pump is directly 
connected to a main. 
Check valves have been installed at intervals along a main leading 
from a pump but have met with little success. Unless provision is 
made to allow some water to escape at the pump when it is suddenly 
